JNTU B.Tech II PHYSICAL METALLURGY Previous Question Paper (October - 2020 - R18)

A phase is defined as a homogeneous portion of a system that has uniform physical and chemical characteristics. It is physically distinct, mechanically separable, and bounded by an interface from other parts of the system.

The limitations of phase diagrams include:

• They represent equilibrium conditions, assuming that the system is at its lowest energy state, which is rarely achieved in practical industrial processes involving rapid cooling or heating.
• They provide no information regarding the kinetics of phase transformations, such as the rate at which a phase forms or the time required to reach equilibrium.
• They do not account for the influence of surface energy or internal stresses on phase stability.
• They generally assume a homogeneous distribution of elements, ignoring micro-segregation or concentration gradients that occur during non-equilibrium solidification.

A eutectic phase diagram is constructed by plotting cooling curves for various alloy compositions across the system. Each cooling curve records temperature versus time as the molten alloy solidifies.

1. Pure Components: Cooling curves for pure metals show a horizontal plateau at the melting point, representing isothermal solidification.
2. Alloy Compositions: For alloys, the cooling curve shows a change in slope at the liquidus temperature (start of solidification) and another at the solidus temperature (end of solidification).
3. Eutectic Composition: At the specific eutectic composition, the cooling curve exhibits a single, long horizontal plateau at the eutectic temperature, where the entire liquid transforms into two solid phases simultaneously.
4. Mapping: By identifying the liquidus and solidus arrest points from multiple cooling curves and plotting them against composition on a temperature-composition graph, the phase boundaries (liquidus, solidus, and solvus lines) are established.
5. Result: The intersection of the liquidus lines at the eutectic temperature and composition defines the eutectic point, completing the framework of the binary eutectic phase diagram.